Hydrostatic power steering systems of the kind shown herein are basically shown in a German publication DE-OS No. 17 51 979 wherein the valve sleeve is axially movable for flow control and distribution of pressure oil to the pressure chambers of a servomotor. Axial movement is effected by means of a coarse pitch thread upon rotation of a steering spindle which rotation also rotates a metering pump through a universal shaft linkage. A torque rod return means is utilized to return the valve sleeve to neutral center position which torque rod is fastened at one end to the steering spindle and valve sleeve and at the other end to the universal shaft linkage. The torque rod is twistedly stressed initially when the steering spindle is rotated by manual force at its steering wheel. Since the first end of the torque rod is rotated while the second end cannot initially rotate due to road resistance against the vehicle wheels, a well known effect, accordingly, the valve sleeve is initially rotated to provide communication between grooves and passages of the valve sleeve and steering housing for pressurizing the servomotor. The return restoring force of the torque rod acts only between the steering spindle and the universal linkage connected to the metering pump. This leaves the valve sleeve uncontrolled by any forces acting to ensure a hydraulically centered neutral position, because once the torque rod has untwisted it is completely without stress which might act to enforce an exact centered position of the valve sleeve. Accordingly, inaccuracy of neutral centering of the valve sleeve can occur due to inherent friction and also as the result of acceleration forces caused by road shock in the course of driving.
The preceding disadvantage is avoided in German Pat. No. 28 14 230 which employs a pair of leaf springs which are bowed and assembled between pressure plates with the convex sides of the springs in contiguity and the ends of the springs engaging the surfaces of the pressure plates. Such assembly is inserted in radial slots which pass through a pair of valve sleeves and upon rotation of one such sleeve compression of the bowed springs takes place whereby stress is stored for return of the valve sleeves relative to each other to a neutral position. Inasmuch as the springs can be inserted in the assembly under some stress, such stress is present for effecting an exact neutral position. The springs are prevented from being overstressed by being sandwiched between the pressure plates against which the compressive forces of valve sleeve rotation are exerted.
In the above described arrangement, the ends of the springs make a sharp edge contact against the pressure plates, and when the springs are compressed or permitted to expand, the change in length causes abrasion and frictional forces to occur. This results in considerable hysteresis in spring characteristic during compression and expansion in the springs during operation of the valve sleeve.
Since the leaf springs of the prior art are disposed radially across the diameter of the valve sleeve, and are narrow in the longitudinal direction of the valve sleeves, their extent of overall flexing is limited by the limited diameter of the valve sleeves. Accordingly, considering that the springs and pressure plates are normally in initial force exerting condition within narrow slots of the valve sleeve, with small surfaces exposed to full restoring force of the leaf springs at all times and more particularly during actuation of the valve, strong forces are brought into being. This could lead to deformation of the valve sleeves and, in any event, increased wear results.
Additionally, background art is seen in U.S. Pat. No. 4,174,612 and in publications of the assignee of this application, entitled:
Der Neue ZF-Servostat, identified as G8470P-SL 3/80d, and PA0 ZF Servostat 2, identified as G8470P-SL 9/80d.